Photomasks are used in a wide variety of applications including the fabrication of semiconductor integrated circuits such as ICs, LSIs and VLSIs. Basically, the photomask is prepared from a photomask blank having a chromium based light-shielding film on a transparent substrate, by forming a predetermined pattern in the light-shielding film by photolithography using UV or electron beams. The current demand for a higher level of integration in the semiconductor integrated circuit market has created a need for a smaller feature size. The traditional solutions are by reducing the wavelength of exposure light and increasing the numerical aperture of lens.
However, since reducing the wavelength of exposure light increases the expenses of equipment and material, it is recommended to avoid such a choice. Increasing the numerical aperture improves resolution at the sacrifice of focal depth, which lowers the process stability and adversely affects the manufacture yield of products. One effective pattern transfer method for solving these problems is a phase shift method. A phase shift mask is used as a mask for transferring a micro-pattern.
Referring to FIGS. 1A and 1B, a phase shift mask, specifically a halftone phase shift mask is illustrated as comprising a substrate 1 and a phase shifter film 2 deposited thereon. The mask consists of a phase shifter 2a that forms a pattern on the substrate and an uncovered area 1a of the substrate 1 that is exposed where the phase shifter 2a is absent. A phase difference of about 180° is set between the light transmitted by the uncovered substrate area 1a and the light transmitted by the phase shifter 2a. Due to light interference at the pattern boundary, the light intensity at the interfering boundary becomes zero, improving the contrast of a transferred image. The phase shift method permits to increase the focal depth for acquiring the desired resolution. This achieves improvements in resolution and exposure process margin, as compared with conventional masks having ordinary light-shielding patterns in the form of chromium film.
Depending on the light transmission of phase shifter, the phase shift masks are generally divided for practical application into full transmission type phase shift masks and halftone type phase shift masks. The full transmission type phase shift masks are transparent to the exposure light wavelength because the light transmittance of the phase shifter section is equal to the light transmittance of uncovered substrate areas. In the halftone type phase shift masks, the light transmittance of the phase shifter section is several percents to several tens of percents of the light transmittance of uncovered substrate areas.
One example of early announced halftone phase shift masks is a multilayer halftone phase shift mask as disclosed in JP-A 4-136854. As shown in FIG. 2, a phase shift film 2 including a metal thin film 3 for controlling transmittance and a transparent film 4 having a sufficient thickness to induce a 180° phase shift to the light transmitted thereby is formed on a transparent substrate 1.
The mainstream intended for commercial application resides in single-layer halftone phase shift masks which are of simpler construction and ensure a precision in the manufacture of photomask blanks and processing into photomasks. As shown in FIG. 3, one known single-layer halftone phase shift mask has a phase shift film 2 (or semi-transmissive film 5) of MoSi base materials such as MoSiO or MoSiON (see JP-A 7-140635). The single-layer halftone phase shift mask is an effective means for accomplishing a high resolution in a simple manner. However, as the light used for exposure becomes of shorter wavelength, a problem arises in mask defect inspection or the like.
Oxynitride films of metal and silicon having a relatively high oxygen or nitrogen content are commonly used as the semi-transmissive film 5 of single-layer halftone phase shift mask blank. They have the nature that their transmittance becomes higher as the wavelength of irradiating light becomes longer. On the other hand, after a semiconductor circuit pattern is written in the blank, the resulting mask must be inspected for defects. The defect inspection system uses light having a longer wavelength than that of a light source for exposure used in the lithography through that mask. For a mask adapted to an ArF excimer laser with an exposure wavelength of 193 nm, for example, the defect inspection system generally uses a wavelength of near 260 nm, typically 266 nm, which is longer than the exposure wavelength 193 nm. For effective defect inspection, a contrast must exist between the uncovered substrate area and the phase shifter. Nevertheless, particularly when the single-layer film is adjusted to a transmittance for an exposure wavelength of 200 nm or shorter, the halftone phase shifter has a considerably high transmittance to the inspection wavelength, preventing defect inspection at a sufficient precision. The problem arising from the difference between inspection wavelength and exposure wavelength is noticed not only in terms of transmittance, but also in terms of reflectance.
It is then desired for a halftone phase shift film to have minimal dependence of transmittance and reflectance on wavelength. One solution to this problem is to construct the halftone phase shift film to a multilayer structure, specifically a halftone phase shift film 2 of the structure in which a film 7 having a phase shift function (typically a silicon oxide and/or nitride film containing a metal) is combined with a metal film 6 having a light absorbing function, as shown in FIG. 4 (see JP-A 7-168343).
Such a halftone phase shift mask is manufactured from a blank comprising a metal film, a phase shift film and a light-shielding film on a transparent substrate, by forming a photoresist film on the blank, patterning the photoresist film, and dry etching the light-shielding film through the resist pattern for transferring the pattern to the light-shielding film. A dry etching technique using a chlorine-based gas is often selected at this etching stage since the light-shielding film is typically formed of a chromium-based material. Then, using the mask pattern transferred to the resist and the light-shielding film as an etching mask, dry etching is performed for transferring the pattern to the phase shift film. A dry etching technique using a fluorine-based gas is selected at the second etching stage since the phase shift film is typically formed of a metal-containing silicon oxide and/or nitride. Subsequent etching is performed on the metal film, completing the pattern transfer to all the layers of the halftone phase shift film. If the etching away of the metal film, which largely affects transmittance, is insufficient, it is impossible to manufacture a mask as designed. On the other hand, an attempt to remove the metal film completely often forces etching into the transparent substrate, which fails to provide a phase difference as designed and hence, a phase shift effect as expected.
In order to perform precise etching of the metal film without causing any damage to the underlying substrate, it is preferred to select an etch-susceptible material as the metal film. However, since the metal film is considerably thin, a choice of an extremely etch-susceptible material may allow the etching of the phase shift film to give damages to the substrate past the metal film. Since the transparent substrate used generally has a relatively high etching rate with respect to the fluorine-based gas used for the etching of the phase shift film, it can be readily damaged if over-etching occurs such that the metal film is completely removed during the etching of the phase shift film.
It is then a common practice to prevent over-etching to the substrate by selecting a material which is resistant to etching with fluorine-based gas for the metal film so that the etching of the phase shift film terminates at the metal film, and selecting a chlorine-based gas as the conditions for etching of the metal film. However, the management of these etching steps is actually very difficult.
For achieving a higher degree of integration, the pattern size of semiconductor integrated circuits becomes finer and finer. The lithographic light source of a stepper (stepping projection aligner) or scanner for transferring the mask pattern to wafers has undergone a transition to a shorter wavelength region than KrF excimer laser (248 nm) and ArF excimer laser (193 nm), with the use of F2 laser (157 nm) being under investigation. Among the photomasks used in the fabrication of semiconductor integrated circuits, phase shift masks are currently on predominant use as the photomask capable of reducing the pattern size. For a further reduction of the pattern size, research works are being made to develop a phase shift mask capable of accommodating exposure light of shorter wavelength.
For such phase shift masks, especially halftone phase shift masks that shift the phase of exposure light transmitted by and attenuate the exposure light transmitted by, the constituent elements, composition, film thickness, layer arrangement and the like of the phase shift film must be selected such as to provide a desired phase difference and transmittance at the short wavelength of exposure light used, for example, the wavelength (157 nm) of F2 laser.
However, if one attempts to acquire a phase difference and transmittance compatible with such shorter wavelength exposure light using the constituent elements of conventional halftone phase shift films used at ordinary wavelengths longer than the F2 laser (157 nm), many characteristics of the resultant halftone phase shift film, for example, etching behavior, etching rate, electroconductivity (sheet resistance), chemical resistance, and transmittance at the inspection wavelength are altered from those at the ordinary wavelengths.
For example, some halftone phase shift films for F2 laser (157 nm) exposure gain a transmittance at the exposure wavelength by resorting to a technique of providing a higher degree of oxidation, such as by increasing the content of oxygen in a film beyond the oxygen level in conventional halftone phase shift films for the ArF excimer laser exposure. Of these, halftone phase shift films of MoSi system, for example, suffer from the problem that the resistance of oxide film to chemical liquid, especially alkaline liquid is unsatisfactory.
It would be desirable to develop a halftone phase shift film that satisfies a phase difference and transmittance upon exposure to shorter wavelength light and that fully clears at a practical level the characteristics required in an overall process from the manufacture of a halftone phase shift mask from a halftone phase shift mask blank to the pattern transfer using the mask.